1. Field of the Invention
The present invention relates to a method of producing an L-amino acid using a bacterium, and more specifically, to a method of producing L-lysine, L-threonine and L-glutamic acid. L-lysine and L-threonine are useful as additives for animal feeds, components of health foods, amino acid infusion, and the like. L-glutamic acid is useful as a food seasoning.
2. Brief Description of the Related Art
L-amino acids are industrially produced by fermentation methods using bacteria belonging to the genus Brevibacterium, Corynebacterium, Escherichia, and the like. Examples of a method of producing L-lysine include the methods described in EP 0643135 B, EP 0733712 B, EP 1477565 A, EP 0796912 A, EP 0837134 A, WO 01/53459, EP 1170376 A, and WO 2005/010175. In these production methods, bacterial strains isolated from nature or artificially mutated strains thereof can be used. Also, bacteria which have been modified by a recombinant DNA technique so that the activity of an L-amino acid biosynthetic enzyme is enhanced have been used.
Methods of modifying the uptake or export of L-amino acids have been known to improve L-amino acid-producing ability. An example of a method of enhancing export of an L-amino acid is a method of producing L-lysine (WO 97/23597) or L-arginine (U.S. Patent Publication No. 2003-0113899) using a bacterium belonging to the genus Corynebacterium which has been modified so that the expression of an L-lysine/L-arginine export gene (LysE) is enhanced. In addition, a method of producing an L-amino acid using a bacterium belonging to the Enterobacteriaceae family which has been modified so that expression of the rhtA gene, rhtB gene, and rhtC gene (JP 2000-189177 A), yfiK gene and yahN gene (EP 1016710 A), ybjE gene (WO 2005/073390), or yhfK gene (WO 2005/085419), have been reported, and it has been suggested that such methods are involved in enhanced export of an L-amino acid.
It is also known that enhancing expression of a gene involved in the uptake of a sugar which is a substrate in fermentation improves the L-amino acid-producing ability. Examples of such a method include producing an L-amino acid using an Escherichia bacterium which has been modified to enhance expression of the ptsG gene (WO 03/04670), and producing an L-amino acid using an Escherichia bacterium modified to enhance expression of the ptsH gene, ptsI gene, or crr gene (WO 03/04674).
The nhaA gene and nhaB gene each encode a membrane protein known as the Na+/H+ antiporter (Biochim Biophys Acta. 2005 Sep. 30; 1709 (3): 240-50.). On the other hand, the nhaR gene is known to encode a regulator that positively regulates expression of the nhaA gene (EMBO J. 1997 Oct. 1; 16(19): 5922-9.). However, whether or not enhancing the activity of the Na+/H+ antiporter has an effect on L-amino acid production has not been previously reported.
The chaA gene is a membrane protein which is known as a sodium-calcium/proton antiporter, but it has never been reported to have an Na+/H+ antiporter function (Biochim Biophys Acta. 2002 Dec. 2; 1556 (2-3): 142-8.). The mdfA gene is known to function as a multidrug/chloramphenicol efflux transporter (J Biol Chem. 2004 Mar. 5; 279(10): 8957-65), but it has never been reported to have an Na+/H+ antiporter function, and the involvement of these genes in the production and accumulation of L-amino acids has not been reported.